KVM: nVMX: Enable nested virtualize x2apic mode

When L2 is using x2apic, we can use virtualize x2apic mode to
gain higher performance, especially in apicv case.

This patch also introduces nested_vmx_check_apicv_controls
for the nested apicv patches.

Signed-off-by: Wincy Van <fanwenyi0529@gmail.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>